1. Field of the Invention
The present invention relates to a power semiconductor device for an igniter having an overheat protection function to protect a semiconductor switching device at an abnormally high temperature in an ignition system for an internal combustion engine.
2. Background Art
An ignition system for an internal combustion engine such as an automobile engine has, as components for generating a high voltage to be applied to an ignition plug, and a power semiconductor device incorporating an ignition coil (inductive load), a semiconductor switching device for driving the ignition coil and a circuit device (semiconductor integrated circuit) for controlling the semiconductor switching device. These components constitute a so-called igniter. The ignition system also has an engine control unit (ECU) including a computer. In such a power semiconductor device for an igniter, the resistance to a load dump surge, i.e., a transient excess voltage surge generated in the power supply voltage, is ordinarily assured as one of items of reliability of the power semiconductor device. In ordinary cases, therefore, a method is used in which the power supply voltage is directly observed and the operation of a semiconductor switching device or an integrated circuit controlling the semiconductor switching device installed in the power semiconductor device is stopped when the power supply voltage is excessively high to protect the integrated circuit.
Electric power for the above-described power semiconductor device for an igniter is ordinarily supplied from a motor vehicle battery. However, fluctuations and surge voltages in the voltage of such a power supply are large. In most cases, therefore, the power supply voltage is clamped with a Zener diode, then regulated in a constant-voltage circuit and supplied into the integrated circuit. Direct observation of the battery voltage requires adding a special signal taking-in terminal and providing a protective device of a large power capacity at the terminal. This means an inevitable increase in manufacturing cost. Further, since the Zener diode is provided on the terminal through which the battery voltage is input for power supply to the integrated circuit, the voltage is fixed substantially at the Zener clamp voltage and the desired sensitivity cannot be obtained with respect to an excess voltage. Thus, the above-described voltage observation method is not suitable for high-accuracy voltage detection.
As a solution to the above-described problem, a technique to protect the above-described switching device by monitoring the current between main terminals of the semiconductor switching device and limiting the control terminal voltage on the semiconductor switching device when the current flowing becomes equal to or larger than a predetermined value has been devised (see, for example, Japanese Patent Laid-Open Nos. 5-259853 and 7-86587).
A technique including forming a thyristor on the substrate on which the switching device is formed to extract a high-potential-side main terminal voltage on the semiconductor switching device when the switching device is off and indirectly monitoring the power supply voltage from the output from the thyristor is also disclosed (see, for example, Japanese Patent Laid-Open No. 2000-183341).